Quantification of Carbon Mineralization with Reactive and Non- Reactive Tracers

Environmental tracers are useful tools for tracking the fate of CO₂ injected and stored in the subsurface, including the quantification of mineralization. Tracers in various ways tend to preferentially partition into gas, liquid, or solid phases and thus allow for the quantification of these processes. Of particular importance are ¹³C and ¹⁴C because they are the only ones tracking the CO₂ molecule itself. The potential of these environmental tracers will be illustrated using laboratory experiments and field studies. One of the case studies we will focus on is the Carbfix site in Iceland. Pure CO₂ and CO₂/H₂S mixtures from the Hellisheidi geothermal power plant near Reykjavik, Iceland were  dissolved downhole in groundwater and injected into a permeable confined basalt formation at about 500m depth below ground. We are using non-reactive (sodium fluorescein, SF₅CF₃, and SF₆) and reactive (¹⁴C and ¹³C) tracers in the project to characterize subsurface CO₂ transport and in situ CO₂-water-rock reactions. In January 2012, 170 tons of pure CO₂ tagged with ¹⁴C and SF₆ were injected followed by 73 tons of a CO₂/H₂S mixture starting in June 2012. Mass balance calculations based on the tracer breakthrough curves suggest that about 90% of the injected CO₂ has been mineralized over a period of only 2 years.